x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 80 insertions, 0 deletions

diff --git a/fs/verity/pagecache.c b/fs/verity/pagecache.c
new file mode 100644
index 000000000..99f5f53ee
--- /dev/null
+++ b/fs/verity/pagecache.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2019 Google LLC
+ */
+#include "fsverity_private.h"
+
+#include <linux/export.h>
+#include <linux/fsverity.h>
+#include <linux/pagemap.h>
+
+/**
+ * generic_read_merkle_tree_page - generic ->read_merkle_tree_page helper
+ * @inode:	inode containing the Merkle tree
+ * @index:	0-based index of the Merkle tree page in the inode
+ *
+ * The caller needs to adjust @index from the Merkle-tree relative index passed
+ * to ->read_merkle_tree_page to the actual index where the Merkle tree is
+ * stored in the page cache for @inode.
+ */
+struct page *generic_read_merkle_tree_page(struct inode *inode, pgoff_t index)
+{
+	struct folio *folio;
+
+	folio = read_mapping_folio(inode->i_mapping, index, NULL);
+	if (IS_ERR(folio))
+		return ERR_CAST(folio);
+	return folio_file_page(folio, index);
+}
+EXPORT_SYMBOL_GPL(generic_read_merkle_tree_page);
+
+/**
+ * generic_readahead_merkle_tree() - generic ->readahead_merkle_tree helper
+ * @inode:	inode containing the Merkle tree
+ * @index:	0-based index of the first Merkle tree page to read ahead in the
+ *		inode
+ * @nr_pages:	the number of Merkle tree pages that should be read ahead
+ *
+ * The caller needs to adjust @index from the Merkle-tree relative index passed
+ * to ->read_merkle_tree_page to the actual index where the Merkle tree is
+ * stored in the page cache for @inode.
+ */
+void generic_readahead_merkle_tree(struct inode *inode, pgoff_t index,
+				   unsigned long nr_pages)
+{
+	struct folio *folio;
+
+	lockdep_assert_held(&inode->i_mapping->invalidate_lock);
+
+	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
+	if (folio == ERR_PTR(-ENOENT) ||
+	    (!IS_ERR(folio) && !folio_test_uptodate(folio))) {
+		DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
+
+		page_cache_ra_unbounded(&ractl, nr_pages, 0);
+	}
+	if (!IS_ERR(folio))
+		folio_put(folio);
+}
+EXPORT_SYMBOL_GPL(generic_readahead_merkle_tree);
+
+/**
+ * fsverity_fill_zerohash() - fill folio with hashes of zero data block
+ * @folio:	folio to fill
+ * @offset:	offset in the folio to start
+ * @len:	length of the range to fill with hashes
+ * @vi:		fsverity info
+ */
+void fsverity_fill_zerohash(struct folio *folio, size_t offset, size_t len,
+			      struct fsverity_info *vi)
+{
+	size_t off = offset;
+
+	WARN_ON_ONCE(!IS_ALIGNED(offset, vi->tree_params.digest_size));
+	WARN_ON_ONCE(!IS_ALIGNED(len, vi->tree_params.digest_size));
+
+	for (; off < (offset + len); off += vi->tree_params.digest_size)
+		memcpy_to_folio(folio, off, vi->tree_params.zero_digest,
+				vi->tree_params.digest_size);
+}
+EXPORT_SYMBOL_GPL(fsverity_fill_zerohash);